Liquid crystal displays are used as displays for computers, displays for the control panels of consumer electronic products and displays for mobile telephones. Desired improvements for liquid crystal displays include achieving lower electricity consumption, lighter weight and thinner form.
A liquid crystal display is not a self illuminating device but must use an external light source or external light from its surroundings. A typical example of an external light source is a back light arrangement whereby a surface light source is disposed at the rear surface of a liquid crystal panel. The back light system requires that light emitted from the surface light source be directed in the frontal direction to the observer.
FIG. 1 shows an example of a configuration of a backlight system. The hologram optical grating 10 is, in the conventional art, a prism sheet. Light obliquely emitted from the light guide plate 12 is bent to a vertical direction at the prism sheet, diffused, at a diffuser 32 to reduce color dispersion before being irradiated to a liquid crystal panel 30 displaying an image. In this backlight system the form of the light guide plate, and the form of the prism sheet disposed between the light guide plate and the liquid crystals are optimized such that the degree of brightness at the front is high.
FIG. 2 shows the angle of incidence θi to and the angle of emission θo from a diffraction grating, however a prism sheet is used for the purposes of the description instead. The angle of emission of light emitted from a light guide plate depends on the design of that plate, but normally the angle of incidence θi is between 20°-70°. The role of the prism sheet is to effectively bend this light such that θo is 0°, in other words, to bend the light in a vertical direction. To do this, it is necessary to reduce Fresnel reflection, that is, reflection at the interface of the atmospheric layer and the prism, and to make the greater part of the light proceed at 0°. Further, when the emitted light is of diverse angles, even if the angle of incidence θi fluctuates to some degree, by having light bending properties that ensure the degree of brightness in the vertical direction does not decrease, the degree of brightness in the frontal direction can be raised, rather than having a constant light bending angle. Moreover, as the light source is white light it is necessary to reduce bend angle wavelength dependence and suppress spectral separation as much as possible. Spectral separation causes a reduction in display qualities such as in the deterioration of color reproduction in the color display of liquid crystals.
A conventional prism sheet uses refraction and total reflection to bend emitted light using geometric optics. In contrast to this, an optical member (hologram optical element) using refraction and interference phenomena based on wave optics, realizes a plurality of functions in a single element, providing a thinner form, and furnishing superior light focus and diffusion characteristics in comparison to optical elements using geometric optics. Such hologram optical elements however, have not been used for bending white light for spectral separation or high order diffraction, but thus far have been employed for diffusing white light and broadening the viewing angle (Japanese Unexamined Patent Application Publication No. 7-114015 (pages 1-2, elected drawing), Japanese Unexamined Patent Application Publication No. 9-325218 (pages 1-2, elected drawing), Japanese Unexamined Patent Application Publication No. 10-506500 (pages 1-4, FIGS. 1-5), Japanese Unexamined Patent Application Publication No. 11-296054 (pages 1-2, FIGS. 2-5), Japanese Unexamined Patent Application Publication No. 2000-39515 (pages 1-2, FIGS. 1-2)), or for spectral separation of white light (Japanese Unexamined Patent Application Publication No. 9-113730 (pages 1-5, elected drawing), and Japanese Unexamined Patent Application Publication No. 10-301110 (pages 1-2, FIGS. 68)). Further, hologram optical elements are being used employing the effects of white light diffusion to make dot matrix display defects invisible (Japanese Unexamined Patent Application Publication No. 5-307174 (pages 1-2, elected drawing), Japanese Unexamined Patent Application Publication No. 6-59257 (pages 1-2, elected drawing), Japanese Unexamined Patent Application Publication No. 6-294955 (pages 1-2, elected drawing), Japanese Unexamined Patent Application Publication No. 7-28047 (pages 1-2, elected drawing) and Japanese Unexamined Patent Application Publication No. 7-49490 (pages 1-2, elected drawing)). For design methods for hologram optical elements please refer for example to “Iterative Methods for Diffractive Optical Elements Computation” by Victor Soifer, Victor Kotlyar and Leonid Doskolvich, US, Taylor & Francis 1997 pages 1-10.
The method of geometric optics theory systems for bending of emitted light presents a problem in that substantial height irregularities mean that sheet film thickness increases, making it difficult to achieve a thin form. Further, in the case of conventional prism sheets, individual prisms perform the function of bending light, and if there are defects on the prism or impurities, light passing that prism may engender display abnormalities such as a luminescent spot of abnormal light rays. A display device of a conventional art being extremely sensitive to defects and impurities, may give rise to display abnormalities that degrade the quality of the product. Accordingly extreme care must be taken in production and handling to ensure there are no such defects and impurities affecting a prism.
Hologram optical elements have the problems that 1) diffractive light other than that of an order of diffraction of vertically refracted incident light arises, 2) the diffractive efficiency of such an order of diffraction is low and 3) wavelength dispersion is substantial. For example if the period is small, there are orders that are not vertically diffracted and wavelength dispersion becomes substantial. If the depth is not appropriate, the diffractive efficiency of such an order of diffraction deteriorates.
With the foregoing in view it is an object of the present invention to use a hologram optical element that utilizes diffraction and interference phenomena based on the wave movement properties of light instead of a conventional prism sheet using refraction, in providing a hologram optical element and a surface light source device using the hologram element that realizes both a high rate of light transmittance in a light bending film and a thin form.